Relax COFF string table check

COFF object files with 0 as string table size are currently rejected. This
prevents us from reading object files written by tools like cvtres that
violate the PECOFF spec and write 0 instead of 4 for the size of an empty
string table.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202292 91177308-0d34-0410-b5e6-96231b3b80d8